Photoelectric converter apparatus

ABSTRACT

In a light receiving element and a semiconductor device manufacturing method, the low density PN junction is formed by constructing the internal composition of the photodiode with N +  type diffusion layer, N -  type epitaxial layer, P -  type epitaxial layer, P +  type deposit layer, and P type Si from the light receiving surface, the vacant layer to be occurred when the photodiode is reverse biased will be widened and the light receiving sensitivity and the frequency characteristic will be improved. Furthermore, since the separation of bipolar elements will be conducted by P -  epitaxial layer, the efficiency in density control at the time of P -  type epitaxial growth can be improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the construction of a light receiving elementand a manufacturing method of the semiconductor device including thelight receiving elements.

2. Description of the Related Art

FIG. 1 shows the construction of a related art of a photodiode and abipolar transistor which are light receiving elements and formed on thesame substrate. In FIG. 1, the left area shows an example of theconstruction of the photodiode, and the right area shows an example ofthe construction of the bipolar transistor. An N type deposit layer 14and a P type deposit layer P⁺ 15 are formed on an n type semiconductorsubstrate 11, respectively. Then a P type semiconductor elementisolation region 17 is formed by growing an N type epitaxial layer 16.

A base region 18 of a P type semiconductor is formed at the right areaon the epitaxial layer 16 and an N⁺ type diffusion layer 19 are formedin the left area and emitter and collector regions in the right area.

Nextly, an electrode layer of photodiode 22 and electrode layers of abase, an emitter and a collector of bipolar transistor 22 are formed inpattern. Then an insulation film is formed thereon. The thickness of theN⁻ type epitaxial layer 16 is determined by the characteristic ofbipolar element, and generally in the bipolar IC having approximately 10V! resisting power, the film thickness of 3-4 μm! was necessary.

According to the construction described above, the light receivingsensitivity of the photodiode is determined by the number of carriersgenerated in a vacant layer 100 and the number of carriers reached tothe vacant layer 100 by diffusion out of carriers generated in the areainterior to the vacant layer. Accordingly, in order to improve the lightreceiving sensitivity it was necessary to widen the vacant layer 100 andto increase the number of carriers to be brought in the vacant layer byproviding semiconductor layer having long diffusion length at the upperand lower parts of the vacant layer 100. These two processes, to enlargethe width of vacant layer 100 and to provide the semiconductor layerhaving long diffusion length lead to control the impurity concentration.

On the other hand, the frequency characteristic of the photodiode isdetermined by the parasitic capacitance and parasitic resistance of thediode. Accordingly, in order to improve the frequency characteristic, itis necessary to decrease the parasitic capacitance and the parasiticresistance. In order to decrease the parasitic capacitance it iseffective to enlarge the vacant layer 1000 and this means the decreasingof the impurity concentration of the junction. Furthermore, to decreasethe parasitic resistance means to increase the impurity concentration ofthe semiconductor layer except the vacant layer. For example, referringto FIG. 1, the P type deposit layer 15 is provided to decrease theparasitic resistance of anode. Since the P type deposit layer 15 hashigh impurity concentration, the lengths of diffusion of a small numberof carriers are short and the carriers which contribute to the lightreceiving sensitivity of the photodiode are almost all carriersgenerated at the upper part of this P type deposit layer 15.

These photodiodes are widely used to read out information recorded onsuch as the compact disc (CD) and mini disc (MD). However, thewavelength of the semiconductor laser used in this type of optical discis 780 nm! and since the absorption length in Si of the laser with 780nm! wavelength is 9 μm!, it created a problem that sufficient lightreceiving sensitivity could not be obtained due to the existence of Ptype deposit layer 15 positioned at 3-4 μm! from the surface in theconventional composition shown in FIG. 1.

SUMMARY OF THE INVENTION

In view of the foregoing, the first object of this invention is toimprove the light receiving sensitivity of photodiodes.

The second object is to improve the frequency characteristics ofphotodiodes.

The third object is to achieve said first and second objects and providea method of manufacturing a semiconductor device which is capable ofeasily isolating photodiodes from bipolar elements.

The foregoing objects and other objects of the present invention havebeen achieved by the provision of a light receiving element in which theimpurity density of the first and second conductive types in the partsto be vacated when impressing the reverse voltage are both kept lessthan 1E16 cm⁻³ !. Thus, since in the case of impressing the reversevoltage to the light receiving elements, the impurity densities of thefirst and second conductive types vacancies to be formed can be keptboth less than 1E16 cm⁻³ !, the vacant layer can be sufficientlyenlarged, and both the improvement in the light receiving sensitivityand the decrease of parasitic capacitance can be achieved. Thus, theconstruction of light receiving element having fairly good frequencycharacteristics can be obtained.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying in which like parts are designated bylike reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a brief sectional view of a photodiode and a bipolartransistor of the related art;

FIG. 2 is a brief sectional view showing the sectional construction of alight receiving element according to a first embodiment of thisinvention;

FIG. 3 is a brief sectional view showing the sectional construction of alight receiving element according to second and fifth embodiments ofthis invention;

FIGS. 4A-4C are brief sectional views showing the process ofsemiconductor manufacturing process according to third and fourthembodiments of this invention; and

FIG. 5 is a brief diagram showing the construction of an optical pickupaccording to fifth and sixth embodiments of this invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Preferred embodiment of this invention will be described with referenceto the accompanying drawings. First, the basic construction will bedescribed.

A brief cross sectional construction of a photodiode PD according to afirst embodiment of this invention will be shown in FIG. 2.

As shown in FIG. 2, P type deposit layer 12 to be formed on the P typesemiconductor substrate 11 is formed selectively in the lower part ofthe photodiode PD. P⁻ type epitaxial layer 13 having an ultra lowdensity is piled on the upper layer and furthermore, over that upperlayer the N⁻ type epitaxial layer 16 is to be piled.

On the other hand, in the bipolar element part, P type deposit layer 15and N type deposit layer 14 are formed between the P⁻ type epitaxiallayer 12 and the N⁻ type epitaxial layer 16. And N⁺ type diffusion layer19 is provided on the cathode surface of the photodiode part.

Since the photodiode PD is constructed as described above, it becomespossible to form the PN junction in the part near the surface like theboundary surface between the P⁻ type epitaxial layer 12 and the N⁻ typeepitaxial layer 16. This means that the vacant layer 100 can be formedon the surface where the quantity of penetrating light is large.

Moreover, in the case of photodiode PD according to this embodiment, theP⁻ type epitaxial layer 12 is formed in ultra low density in order toincrease the light receiving sensitivity of the photodiode PD, andnormally the N⁻ type epitaxial layer 16 is set to less than 1E16 cm⁻³ 3!density as the density to optimize the bipolar characteristic.

Accordingly, the vacant layer spreading centering around the PN junctioncan be sufficiently extended toward both the P type semiconductor layerside and the N type semiconductor layer side. Moreover, since in thecase of impressing the reverse voltage, the N⁻ type epitaxial layer 16and the P⁻ type epitaxial layer are deposited in the part where thevacant layer 100 is to be extended and the P type deposit layer 12 andthe N type diffusion layer 19 are provided in the part where the vacantlayer 100 would not reach, the width of the vacant layer 100 can besufficiently extended and as a result, the parasitic capacitance can bedecreased.

Furthermore, regarding the separation of bipolar elements, since the P⁻type epitaxial layer is placed under the P type deposit layer 15, the Ptype deposit layers 15 can be connected via this epitaxial layer, andthe P type deposit layer 12 for connecting the P type silicon substrate11 and the P type deposit layer 15 as the construction shown in FIG. 1becomes unnecessary. The advantage of placing the P⁻ type epitaxiallayer under the P type deposit layer 15 also exists on the photodetectorPD side. Since it becomes unnecessary to connect the P type siliconsubstrate 11 and the P type deposit layer 15 by the P type deposit layer12, the restriction imposed on the thickness of P type deposit layer 12can be removed.

Then, FIG. 3 shows a brief sectional view of the photodiode PD accordingto a second embodiment of this invention.

As shown in FIG. 3, this photodiode PD comprises the P type epitaxiallayer 13 consisting of P type epitaxial layer 13-1 having almost equalconcentration to the P type silicon substrate 11 and P type epitaxiallayer 13-2 with ultra low concentration, and the P type epitaxial 13-1is in contact with the P⁺ type deposit layer 15.

Moreover, in the case of impressing the reverse voltage, the vacantlayer 100 completely vacates the P⁻ type epitaxial layer 13-2.

In the P⁻ type epitaxial layer 13 which works as isolation of thebipolar element, its latch up resistivity can be increased the higherthe density is. Accordingly, to increase the density of the part not tobe vacated in the P type epitaxial layer 13-1 as compared with the partto be vacated in the P⁻ type epiaxial layer 13-2 contributes todecreasing the parasitic resistance caused by the unvacated P⁻ typeepitaxial layer 13-1 and makes the improvement of the latch upresistivity possible as well as maintaining the feature of low parasiticcapacitance which is the feature of this invention shown in FIG. 2.

In summing up, since the impurity densities of the parts to be vacatedin case of adding the reverse voltage to the photodiode are set to lessthan 1E16 cm⁻³ ! both on the P type side and the N type side, the vacantlayer 100 can be enlarged sufficiently and photo receiving sensitivitycan be improved and further the frequency characteristic can be improvedby decreasing the parasitic capacitance.

Furthermore, at this point, since the part containing higher densitythan the part vacated is provided in the parts other than the part to bevacated at the time when the reverse voltage is impressed on thephotodiode PD, the decrease of the parasitic capacitance can beachieved.

Furthermore, since it becomes unnecessary to connect the P type depositlayer 12 for the bipolar element separation and the P type deposit layer15 like the structure shown in FIG. 1 and since the P type deposit layer12 in the lower part of the bipolar element becomes unnecessary, itbecomes sufficient to have a small opening area of the P type depositlayer 12 and the density of the P⁻ type epitaxial layer 13 can becontrolled easily at the time when depositing the ultra low density P⁻type epitaxial layer 13.

Nextly, an element forming method according to a third embodiment ofthis invention shown in FIG. 2 will be described.

The manufacturing process of the photodiode PD described in thepreceding chapter will be explained in the following chapter referringto FIGS. 3 and FIGS. 4A-4C.

As shown in FIG. 4A, firstly, the P type silicon substrate 11 is thermaloxidized and an oxidized film with the thickness of approximately 120nm! is formed on its surface. Then, making the photoresist as a mask theboron is ion implanted at the temperature of 30 keV! to the photodiodeunit selectively under the condition of 2.4E15 cm⁻² !.

Then, in order to passivate the ion implanted boron, the passivationannealing will be conducted for 80 minutes in the atmosphere of N₂ of1200 °C.!.

Furthermore, in order to remove the drawback caused by the damage whenimplanting the ion, by oxidizing in the atmosphere of wet O₂ for 20minutes at 1200 °C.! P type deposit layer 12 is formed. Then by removingthe oxide layer by using hydrofluorite acid, the construction of FIG. 4Awill be obtained.

Then, the P⁻ type epitaxial layer 13 is deposited under the condition of15 μm! and 20 Ω.cm!. Then after attaching 100 nm! oxide film by thethermal oxide film, the N type deposit layer 14 will be formed by ionimplanting the phosphorus at 50 keV! under the condition of 8E14 cm⁻² !selectively to the bipolar element part making the photodiode PD as amask. Then, the P type deposit layer 15 will be formed by ion implantingthe boron at 30 keV! under the condition of 2.5E15 cm⁻² ! making thephotodiode PF as a mask.

Furthermore, in order to passivate the ion implanted ion, a passivationannealing will be conducted for 80 minutes in the atmosphere of 120 °C.!N₂. Then in order to remove the drawback due to the damage occurred whenimplanting the ion, oxidation will be conducted for 20 minutes in theatmosphere of 120 ° C.! wet O₂. Then, the construction of FIG. 4B willbe obtained by removing the oxide film using hydrofluorite acid.

Then, the N⁻ type epitaxial layer 16 will be piled under the conditionof 4 μm!, 1 Ω.cm!.

Moreover, after adding a 10 nm! oxide film by the thermal oxidation,boron is ion implanted at 50 keV! under the condition of 5E15 cm⁻² ! tothe isolation unit of bipolar element and the anode pull out unit of thephotodiode PD making the photodiode PD as the mask, the P type isolationlayer 17 will be formed. At this point, by conducting the passivationannealing in the atmosphere of N₂ for 80 minutes at 110 °C.!, the P typeisolation layer 17 will be connected to the P type deposit layer 15 andthe isolation process of the bipolar element will be conducted. As aresult, the construction of FIG. 4C will be obtained.

Then, in order to form a P type base 18 on the NPN transistor part,boron is 1E14 cm⁻² ! ion implanted at 30 keV! and the passivationannealing is conducted for 30 minutes in the atmosphere of 900 °C.! N₂.Then, BF₂ is 1E15 cm⁻² ! ion implanted at 50 keV! in order to form Ptype diffusion layer 20 forming the contact part with the metal panel ofthe base of NPN transistor and the contact part with the metal panel ofthe anode of the photodiode PD. Moreover, arsenic is 5E15 cm⁻² ! ionimplanted at 50 keV! and the passivation annealing is conducted for 20minutes in the atmosphere of N₂ in order to form N type diffusion layer19 at the contact part of an emitter and collector of the NPN transistorand the metal panel, and on the cathode surface of the photodiode.

Then, 600 nm! silicon oxide film is deposited as a film between thefirst layer AL metal panel and Si layer using the CVD method, and acontact hole will be opened using the RIE (reactive ion etching) inorder to get the ohmic contact among the first layer AL panel and thebipolar element and the photodiode PD. Then, Ti and TiON, i.e., metalswith high melting point, are deposited with the film thickness ofapproximately 30 nm! and 70 nm! respectively by the sputtering method.Moreover, 600 nm! of A1 containing 1 %! of Si having low melting pointis deposited by the sputtering method. Then, unnecessary part of themetal panel 22 is removed by the RIE etching method.

Then, 1 μm! silicon oxide film will be deposited by the plasma CVD(chemical vapor deposition) method as a film 23 between the first layerAL panel 21 and the second layer AL panel 24 and the then the levelingprocess with the SOG (spin-on-glass) is conducted, and then the siliconoxide film will be deposited by the plasma CVD method. Then, the contacthole to connect the first layer AL panel and the second layer AL panelis opened by the RIE method, and 100 nm! and 1000 nm! of Ti and AlSi (1percent) are piled respectively by the sputtering method.

Then, these metal layers only in the photodiode unit are removedselectively by the RIE etching method. And 700 nm! of siliconnitridefilm is piled as an over passivation film 25 by the plasma CVD methodand the siliconnitride of the bonding pad part is removed by the RIEetching method. Then, the thermal processing for sintering is conductedat 400 °C.! for 30 minutes in the atmosphere Fo gas, and as a result,the construction shown in FIG. 2 can be obtained.

Furthermore, a manufacturing method according to a fourth embodiment ofthis invention shown in FIG. 3 may be conducted by continuously pilingthe condition of said P⁻ epitaxial layer 13 under the conditions of 5μm!, 4 Ω.cm! and 10 μm!, 20 Ω.cm!.

An application example according to a fifth embodiment of this inventionwill be described.

The construction of an optical pickup as an application example of thedevice using the photodetector PD will be shown in FIG. 5. This opticalpickup is formed by mounting a prism 32 comprising optical coupler onthe optical IC substrate 31. The photodiode PD having the constructionas described above is deposited on the optical IC substrate 31, and theoptical coupler and the photodiode PD are kept on the positions havingthe fixed position relation.

Moreover, at the one end of the optical IC substrate 31, semiconductorchip 33 equipped with semiconductor laser LD is to be mounted inaddition to a prism 32. In this semiconductor chip 33 photodiode PD_(M)for detecting the light emitted backward from the semiconductor laser LDand controlling the power of semiconductor laser LD (e.g., for powermonitor with PIN construction) is formed. The above is the constructionof an optical pickup.

According to the foregoing construction, an outline of the opticaldetection operation of the optical pickup will be described below.Firstly, after the light generated from the conductor laser diode ID isreflected, condensed on an optical disc 35 in which bit information isstored via a lens system 34. This light is modulated corresponding tothe information recorded on the optical disc 35 and reflected, and thenfed into the prism 32 from the incident surface 32A of the prism 32.

Then the ray inputted will be reflected in the prism 32 and condensed onmultiple photodiodes PD placed under the prism.

Since all photodiodes PD constructed according to the foregoingconstruction have high photo-receiving sensitivity and the degradationof frequency characteristic is small, information on the optical disc 35can be read out completely even in the case where it is difficult tolead the sufficient quantity of light on the photodiode PD due to thecomplicated optical system being utilized in these days.

Furthermore, as the photo receiving result of multiple photodiodes PD,the read out of recording information, tracking servo, and focus servocan be executed.

Nextly, a sixth embodiment of this invention will be described.

In the embodiment described above, an optical pickup is used as apractically applied device. However, this invention is not only limitedto this but also can be applied to various devices having thephotodetector built-in as the photo receiving element.

Furthermore, in the embodiments described above, it is limited to the Ptype deposit layer 12 to deposit under the photodiode PD. However, thisinvention is not only limited to this but also it may be deposited underthe bipolar transistor element. With this arrangement, the patterningprocess for depositing the P type deposit layer 12 under the photodiodePD can be omitted.

Furthermore, regarding the condition of ion implantation and theannealing condition, this invention is not only limited to theembodiments described above but the other values can be adoptedaccording to the processing condition required.

According to this invention as described above, since the photoreceiving element is to be constructed making the impurity densities ofthe first and second conductive type in the vacant part to be formed incase of impressing the reverse voltage both less than 1E16 cm⁻³ !, thevacant layer of the photo receiving element can be sufficiently enlargedand an improvement of photo receiving sensitivity as well as thedecrease of parasitic capacitance can be realized. Thus, the photoreceiving element having an excellent frequency characteristic can beformed.

While there has been described in connection with the preferredembodiments of the invention, it will be obvious to those skilled in theart that various changes and modifications may be aimed, therefore, tocover in the appended claims all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:
 1. A photoelectric converter apparatus having abipolar element side and a photodiode side, comprising:a semiconductorsubstrate of a first conductive type; a first semiconductor layer of thefirst conductive type formed over a portion of said semiconductorsubstrate, said first semiconductor layer used in said photodiode sideonly; a second semiconductor layer of the first conductive type formedover both said first semiconductor layer and said semiconductorsubstrate; a third semiconductor layer of a second conductive typeformed over a portion of said second semiconductor layer on said bipolarelement side; a fourth semiconductor layer of the second conductive typeformed over both said second semiconductor layer and said thirdsemiconductor layer; and an electrode formed on said fourthsemiconductor layer.
 2. The photoelectric converter apparatus as claimedin claim 1, wherein an impurity concentration of said secondsemiconductor layer is less than an impurity concentration of said firstsemiconductor layer.
 3. The photoelectric converter apparatus as claimedin claim 1, wherein an impurity concentration of said fourthsemiconductor layer is less than an impurity concentration of said thirdsemiconductor layer.
 4. The photoelectric converter apparatus as claimedin claim 1, further comprising:a fifth semiconductor layer formedbetween said second semiconductor layer and said third semiconductorlayer.
 5. The photoelectric converter apparatus as claimed in claim 4,wherein said fifth semiconductor layer is of the first conductive type.6. The photoelectric converter apparatus as claimed in claim 5, whereinan impurity concentration of said fifth semiconductor layer is greaterthan the impurity concentration of said second semiconductor layer. 7.The photoelectric converter apparatus as claimed in claim 1, wherein avacancy layer is formed from said second and third semiconductor layersin an upper region of said first semiconductor layer, and wherein theimpurity concentrations of said second and third semiconductor layersare both less than 1E16 cm⁻³ ! in vacancies to be formed in the case ofimpressing a reverse voltage.
 8. The photoelectric converter apparatusas claimed in claim 7, wherein the impurity concentrations ofnon-vacancy layer semiconductor layers are greater than impurityconcentrations of vacancy layer semiconductor layers.
 9. Thephotoelectric converter apparatus as claimed in claim 7, whereinnon-vacancy layer semiconductor layers are formed at a distance from asurface of the apparatus which is greater than the absorption length oflight to enter said vacancy layer.